With reference to FIG. 11, a shutter mechanism of the prior art includes a very thin flexible steel foil 12 which may be deflected into a beam path 14 by an electromagnet 16. The foil 12 is attached at one end to a collar 18 by retaining screws 20 and lies parallel and below the beam path 14 when the electromagnet is inactive. The electromagnet 16 comprises a ferrite core 22 surrounded by a magnetic winding 24 on a pancake bobbin. In operation, the winding 24 is energized, activating the magnet 16 and causing the free end 26 of the foil 12 to be attracted to the magnet 16 and move upward. As the end 26 intercepts the laser beam path 14, the beam is reflected away from the path 14 by a few degrees. The angle of reflection increases as the foil end 26 approaches the magnet 16. In the full closed position seen in the drawing, the foil 12 conforms flat near magnet 16, bends sharply down to the retaining screws 20 and intercepts the laser beam at a 50 to 60 degree incidence angle. A heat sink 25 removes heat generated by electromagnet 16.
The very thin foil 12 is not only extremely flexible, allowing it to bend into an "S" shape when blocking the beam, but also has low mass which aids in reducing vibration caused by the collision of the foil 12 and the electromagnet 16. This is an advantage because the attenuator or shutter is commonly fitted over cylindrical housings standard for helium-neon and other gas lasers, and any vibration is easily coupled. However, the thin foil is inherently weak, especially at stress points in the "S" shaped bend and at notched cutouts for the retaining screws 20. The lifetime of foil 12 is about 100,000 cycles before breakage occurs, limiting its use to low speed on/off operation. Thermally, the thin foil is not capable of conducting away sufficient heat arising from absorption of high power laser light, but is suitable for low power laser attenuation.
The foil 12 reflects the light beam at an angle near that of the unaltered beam path 14. Thus, an unwanted stray reflection line appears at the target plane of the laser beam as the foil end 26 begins to intercept the beam. It occurs from initial interception until the foil 12 flattens against the electromagnet 16. Since the reflection is at a very narrow angle with respect to the unaltered beam path 14, the reflection cannot be eliminated in the device, but must be "trimmed off" somewhere between the attenuator and the target plane. For low speed on/off operation this poses little problem. But cycling the attenuator or shutter in a high speed pulse operation would produce a large continuous "streak" reflection which is unacceptable. Further, even in low speed operation, the orientation of the foil in the fully closed "S" shape allows laser light to be scattered back into the laser. This back scattering is undesirable in applications such as interferometry.
An object of the present invention is to produce a laser beam shutter mechanism having long life and no stray reflection so as to be suitable for high speed pulse operation.
Another object of the present invention is to produce a shutter mechanism suitable for use with high power lasers and sources.